There is no evidence to suggest herbal medicines "tailored" to the individual work, and they may even do serious damage, according to a study.
'No convincing evidence' that mixing these up makes you feel better
Scientists writing in the Postgraduate Medical Journal examined what they said were the only three clinical trials to have been conducted on the treatments.
They expressed doubts as to the skills of those in the UK who offer treatments specially formulated for individuals.
But UK herbal practitioners said such treatments can make a real difference.
The UK is currently reviewing the law in relation to the regulation of this field, so at present it is unclear how many such practitioners there are in the country.
Drawing on Chinese and European traditions among others, the practitioners offer a wide variety of treatments for conditions ranging from minor skin ailments to cancer, using a multitude of herbs.
The team from the Peninsula Medical School, a partnership between Exeter and Plymouth universities and the NHS in Devon and Cornwall, stressed that there were many herbs with health benefits, but that studies on these tended to involve standard preparations or single herb extracts.
They said they searched widely for randomised clinical trials of tailored treatments across the world in any language and contacted 15 professional bodies in the process, but were only able to find three trials.
One compared a tailored Chinese herbal preparation with a standard herbal preparation and a placebo for irritable bowel syndrome (IBS).
The other two compared tailored treatments with placebos for chemotherapy-related toxicity in cancer patients and osteoarthritis respectively.
There were no statistical differences between tailored and placebo in either the osteoarthritis or cancer study.
Tailored treatment did seem to work better than the placebo in IBS, but it was not as effective as the standard treatment.
And while these trials did not suggest that patients had suffered ill health as a result, Dr Peter Carter said there were serious risks attached to tailored treatment.
These included herbs interacting negatively with each other - a much greater risk with tailored treatment than retail herbal remedies given the number of herbs used in preparations - as well as with prescription drugs.
Herbs could be contaminated or even toxic, and their strength misunderstood by the practitioner.
"There are lots of issues around expertise," said Dr Carter.
"Can the practitioner make a proper judgement, do they know when a client is displaying symptoms that really should be seen by a doctor?"
A spokesperson for the National Institute of Medical Herbalists said it was impossible to draw conclusions from three small studies with "questionable methodology", and that herbalists often found themselves unable to obtain the funding necessary to carry out rigorous trials.
"These treatments really can make a difference to people's health - that's why they have been used for such a long time," said Alison Denham.
"People often come to us having gone down the orthodox road with no luck, and find something that works for them.
"But there are certainly issues around the expertise of practitioners which need to be addressed, and we look forward to government regulation which imposes a high standard of training on anyone who wants to register."